The present invention relates to data communication between an aircraft and a ground-based destination device. More particularly, the present invention relates to a method and apparatus for enhancing the current data communication capabilities of the North American Terrestrial System (NATS) Network.
Current data service offerings to commercial aircraft are limited in function and capability. One of the known systems for transmitting data to and from an aircraft is the Aircraft Communication Addressing and Reporting System (ACARS). The ACARS system is primarily used to transmit two types of data. The first type of data is known as safety data. Safety data is transmitted from the pilot/flight crew to the tower or the dispatch and includes information such as the aircraft's heading and the aircraft's altitude. The second type of data is known as operational/administrative data. Operational/administrative data is transmitted from the aircraft to the dispatch and may include, for example, information which indicates when particular equipment aboard the aircraft is in need of repair.
The ACARS system typically comprises an ACARS management unit (MU) and a very high frequency (VHF) transceiver. In general terms, the ACARS MU collects aircraft data from various sources aboard the aircraft (such as avionics equipment) and sends the collected data to the VHF transceiver. The VHF transceiver encapsulates the data in accordance with a protocol, modulates a frequency channel using the encapsulated data, and transmits the modulated frequency channel to the control tower or the dispatch. Upon successfully receiving data, the tower or the dispatch uses a frequency channel to transmit an acknowledgment message to the originating aircraft.
There are several drawbacks associated with the ACARS system. First, the number of channels available for transmitting data is limited. The geographic region of the United States is subdivided into a number of Air Traffic Control (ATC) regions. Each ATC region is allocated only one channel for ACARS system transmissions. Thus, each ACARS equipped aircraft within a particular ATC region must share the same channel.
A second drawback is that the ACARS system is limited to transmitting short messages. The system is not suitable for transmitting files of data to and from an aircraft. Third, the ACARS system is not intended for public correspondence. Thus, passengers aboard an aircraft are unable to use ACARS to transmit data from the aircraft.
Another drawback associated with the ACARS system is that the system lacks an encryption method. Some of the data transmitted to and from the aircraft is considered sensitive. One example of such sensitive data is aircraft performance data. Aircraft performance data may include parameters such as engine vibration, engine temperature, and whether or not the tail of the aircraft was dragged across the ground during a take-off or a landing. This type of information is considered sensitive because it could be used to indicate that the pilot did something wrong while flying the aircraft.
Finally, the ACARS system implements protocols which, although widely used within the aviation community, are incompatible with commercial “off the shelf” software and protocols. The use of these protocols makes it difficult to develop new application software to run over the ACARS data link. Further, system enhancements and upgrades are difficult because there are a limited number of software developers/engineers familiar with how the ACARS protocols are implemented.
Another known system which provides limited data communication between an aircraft and a ground station is known as the North American Terrestrial System (NATS) Network. The NATS Network comprises a NATS radio set aboard each subscribing aircraft and more than 150 AT&T Corp. NATS ground stations strategically located throughout the continental U.S., Canada, and Mexico in order to provide full coverage to subscribing aircraft at cruise altitude. Each of the over 150 ground stations are connected to both a public switched telephone network (PSTN) and a wide area network (WAN). In addition, micro-cell ground stations are deployed at key airports to provide localized ground coverage to aircraft on the tarmac or at the gate.
Aboard each subscribing aircraft, a plurality of telephone handsets are adapted to be connected to each NATS radio set. These telephone handsets are adapted to facilitate connecting a device such as a laptop computer to the NATS radio set. Passengers may use the telephone handsets to exchange voice information with ground-based telephones via the NATS radio set, the ground stations and the PSTN. Similarly, passengers may exchange data with ground-based systems via the NATS radio set, the ground stations and the WAN.
The NATS radio set operates in the UHF range. The air-to-ground transmit band is located between 894 and 896 MHZ. The ground-to-air band is between 849 and 851 MHZ. Each band is broken into 10 channel blocks which contain 29-6 kHz wide user channels (one for uplink and one for downlink). Additionally 63.2 KHz wide control channels (a.k.a. pilot channels) are available in each channel block. Each ground station is assigned one of the 10 channel blocks. These pilot channels are used by the ground station to broadcast frequency availability and sundry other status and control messages to the airborne radios. One of the six available pilot channels is assigned to each service provider. Thus, aircraft flying over a geographic region served by a particular ground station may exchange voice and data information with ground-based systems using one of the 29 channels available to that ground station.
Unfortunately, the NATS Network is primarily intended to facilitate voice (rather than data) transmission to and from an aircraft. NATS voice and data transmissions are implemented in accordance with a technique known as circuit mode. In circuit mode, one telephone handset or one laptop computer connected to a telephone handset aboard a subscribing aircraft may exchange voice or data information with one ground-based destination device using one available NATS channel. Each user aboard an aircraft is assigned one channel for the duration of the time they are transmitting and receiving information.
As a subscriber aircraft flies over a geographic region, the NATS radio set continuously monitors the signal strength from the available ground stations and transmits data to the ground station which is currently providing the strongest signal strength. In circuit mode, the process of switching ground stations breaks the transmission link between the aircraft and the ground station. Thus, data transmission is interrupted during the switching process.
Circuit mode transmission may be contrasted with a technique known as packet mode. In accordance with packet mode, multiple users aboard an aircraft might share a singe channel to transmit data to multiple different destination addresses. Due to circuit mode implementation and other NATS Network constraints, only a limited number of users aboard any one aircraft may simultaneously make telephone calls from the aircraft and only a limited number of users aboard any one aircraft may simultaneously use their laptop computers to transmit and receive data from the aircraft.
Thus, there is a need for a method and apparatus which solves the above described problems.